1. Field of the Invention
The present invention relates to a liquid crystal display and more specifically, to an apparatus for detecting abnormal states in a high-voltage cable connected to a discharge tube of a liquid crystal display.
2. Related Art
Liquid crystal displays are used in portable computers, that is, notebook type or subnotebook type computers, principally because they are smaller in size and consume smaller quantities of current than a CRT. However, since there is a need to further downsize the computer itself for improved portability and as there are demands to use as large a screen as possible even with a portable computer, even the parts used in a liquid crystal display must also be downsized further. Thus, various schemes have been tried with the backlight used in a liquid crystal display. For example, the inverter was formerly loaded on the liquid crystal display side of the computer to minimize the length of high-voltage cable, but recently there are some constructions in which the inverter is loaded on the computer body side and the high-voltage cable passes through a hinge between the liquid crystal display and the computer body.
FIG. 9 shows one such example. A computer 1 includes a computer body 3 and a liquid crystal display 9. The computer body 3 includes a keyboard 7, a floppy disk drive 5, and a CPU, memory, hard disk drive and the like not shown here. This computer body 3 is connected to the liquid crystal display 9 by using hinges 11a and 11b. The liquid crystal display 9 delineates images on a liquid crystal display panel 13 and conveys the results processed in the computer body to a user. Simply put, what is normally termed a backlight comprises a discharge tube 15 and a light conducting panel and a diffusing panel on the back of the liquid crystal display panel 13. That is, one or more discharge tubes 15 are provided vertically as shown in FIG. 9, vertically at two sides of the display panel 13, or horizontally at one side or two sides of the display panel 13, and rays of light from the discharge tubes are conveyed via the light conducting plate and the diffusing plate to the whole liquid crystal display panel 13, so that the liquid crystal display panel 13 can be seen brightly. Incidentally, the value of voltage applied to the discharge tubes 15 may, for example, be on the order of 1200V at the start and 500V upon lighting.
As mentioned above, because conventional liquid crystal display panels 13 were formerly small compared to the size of the liquid crystal displays 9, etc., an inverter was also provided in the liquid crystal display 9. However, as shown in FIG. 9, an inverter 19 is now being provided in the computer body 3 by extending a high-voltage cable 17 to pass through the hinge 11b. With such a structure, the portion indicated by the circle A may be hazardous. That is, as the cable is subjected to repeated stress due to the movable hinge portion 11b, the core wire may consequently break.
There may also be cases where a structure must be used in which a high-voltage cable has to be passed not only through the area of circle A but also through an area where there is a great possibility of the cable being pinched by a screw or the frame.
In such cases, there may be situations where there are discharges at the break in the wire or the wire insulation may be torn so that there are discharges toward a screw or the frame. Although it is easy to make the cable difficult to break and the insulation hard to tear, these are not fundamental solutions to the problem.
Here, the construction of a conventional inverter will be described in reference to FIG. 10. A discharge tube 15 is connected through a ballast capacitor 23 to the secondary winding side of the transformer 21. To detect current flowing through this discharge tube 15, a tube current detection section 25 is mounted. The value of current detected in this tube current detection section 25 is fed back to keep the tube current constant. Here, a description of how the fed back value of current is used to keep the tube current constant will be omitted because it is not directly related to the gist of the present invention.
If a situation should occur where current is consumed by portions other than the discharge tube due to discharge or the like and the current flowing through the discharge tube 15 consequently decreases, the decrease in tube current causes the feed back to decrease so that it is possible to detect the above situation. However, the following problems have not yet been solved: 1) usually, if the tube current decreases, a positive feedback acts in such a manner to increase the tube current and consequently output increases. Thus, current increases and continues to flow until such a safety circuit as a fuse operates; 2) There is a darkening effect (when a cold cathode tube is lit after leaving it for some time in the dark, the lighting is delayed sometimes for several seconds or tens of seconds because the number of initial electrons is small) and, since output cannot be stopped even if no tube current flows directly after the start, the output must be continued for a while and no countermeasures whatever can be taken while the output continues. For example, when the above situation occurs in portion B of FIG. 10, no drastic countermeasures can be taken with the above conventional method.
Published Unexamined Patent Application (PUPA) No. 6-20779 for example, describes an arrangement for detecting abnormal states where either of two fluorescent tubes provided does not light, but nothing about how to deal with cases where only one fluorescent tube is provided nor how to handle the occurrence of a discharge or the like. PUPA No. 5-343187 describes an arrangement for detecting abnormal states at a place where an eddy current flows when a short circuit/open circuit occurs on the primary side of a transformer, but no countermeasures whatever against such abnormal states at the secondary side as discharge due to a high voltage. PUPA No. 4-342991 describes an arrangement provided to deal with an acoustic resonance phenomenon but nothing about countermeasures against discharge or like circumstances and further, since the reference voltage for detecting abnormal states is fixed, no countermeasures can be taken when the darkness effect is acting. Furthermore, PUPA No. 6-86454 discloses a structure for detecting the short circuit of a load but takes no account of such circumstances as discharge and no countermeasure can be taken when the darkness effect is present because abnormal states are detected by using an input voltage to the load as the reference voltage.